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Microcystis aeruginosa is among the most common harmful algal-blooming species in the world. Potent microcystins released by M. aeruginosa have been linked to liver failure and death in aquatic mammals, like the endangered California sea otter, and provide a serious public health risk to humans. Once characterized as a freshwater problem, M. aeruginosa is expanding on a global scale, making persistent returns in freshwater, brackish, and coastal marine ecosystems. Though commonly observed dominating aquatic ecosystems in low N:P atomic ratios less than 44:1, the reliability of N:P ratios as a tool for managing and predicting M. aeruginosa blooms is explored through analyzing the specific roles of N and P in influencing its growth, toxicity, and distribution. N is far more influential than P in regulating growth, toxicity, and distribution of M. aeruginosa, and ammonium (NH4+) is the most bioavailable form of N to M. aeruginosa. Thus, as the proportion of NH4+ increases in total N concentrations, phytoplankton communities shift towards M. aeruginosa dominance. Changing concentrations of N and P have been observed to influence the N:P ratio values that provide optimal growth, with higher N and P concentrations providing a higher range of N:P ratios in which optimal growth can occur. Furthermore, local environmental factors can influence M. aeruginosa presence more than nutrient availability, with M. aeruginosa favoring extended dry seasons and warmer surface water temperatures between 25-30°C. Management of M. aeruginosa should focus on regulating concentrations of N and P, including NH4+, instead of using N:P ratios as a management tool. Furthermore, management decisions may be unique depending on local environmental factors.